You may also like

How can you put five cereal packets together to make different
shapes if you must put them face-to-face?

Semaphore Signals

Stage: 2 Challenge Level:

Many of you sent in solutions to the first
part of this question, correctly saying that the message reads
"hello".

Yanqing who goes to Devonport High School for
Girls wrote to say:

I think there are 7+6+5+4+3+2+1=28 ways altogether:

There are 8 possible positions for a flag but you can't put
both flags in the same position, so when one of the flags is in
position 1, there are 7 positions where you can put the second
flag; when one of the flags is in position 2, because we have
already counted "one flag in 1 and the other in 2", we have 6 new
positions; when one of the flags are in position 3, we have 5 new
positions; ... and so on.

When we get to: one of the flags is in position 8, all of the
positions have already been counted, so we have:
7+6+5+4+3+2+1.

The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the
NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to
embed rich mathematical tasks into everyday classroom practice. More information on many of our other activities
can be found here.